As described in the article "Elastomers, Synthetic", Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 7, pages 677-716 (Interscience Publisher 1965), "Synthetic elastomers are a group of synthetic high polymeric materials with properties that, in the past, might have been described as rubbery." The ASTM definition for rubber (1964 Book of ASTM Standards, ASTM D1566-62T) appears to also cover elastomers. This definition, which is based on physical characteristics and not on chemical structure, defines a rubber as "a material that is capable of recovery from large deformations quickly and forcibly and can be, or already is, modified to a state in which it is essentially insoluble (but can swell) in boiling solvents such as benzene, methyl ethyl ketone, and the ethanol-toluene azeotrope. A rubber in its modified state, free of diluents, retracts within one minute to less than 1.5 times its original length after being stretched at room temperature (20.degree.-27.degree. C.) to twice its length and held for one minute before release."
With a few exceptions, elastomers are not typically employed in their raw or dry state. For the great majority of uses, the elastomer must be modified, usually by the addition of crosslinking or curing agents, followed by a heat cure.
One common elastomer of general utility is ethylene-propylene-diene monomer terpolymer (EPDM). EPDM polymers are desirable elastomers since they are prepared from low-cost monomers and have good mechanical and elastic properties as well as outstanding resistance to ozone, heat and chemical attack.
EPDM polymers may be sulfur-cured but usually require ultra-accelerators in the recipe because of the low-polymer unsaturation. EPDM can be vulcanized with systems based on sulfur, peroxides, quinoids, or polyhalomethyl resins. Some of the best current systems contain sulfur (1.5 phr), zinc oxide (5 phr), stearic acid, a primary accelerator (thiuram mono-, di- or tetrafulfides or metal salts of a dithiocarbamic acid (1.5 phr)), and a thiazole (0.5 phr) as a secondary accelerator. High oil and black levels can be accepted. With recipes such as these, satisfactory cures for some applications may be obtained in the usual times and temperatures (e.g., 30 min. at 320.degree. F.).
Peroxide curing of EPDM elastomers is the most common system. Peroxide curing involves coupling of allylic radicals derived from the cure site monomer and peroxide generated radicals to give vulcanizates containing allylic carbon-carbon bonds. This curing system is not desirable for certain applications. For example, when such EPDM elastomers are used as seals in high temperature applications (above about 300.degree. C.), the peroxide-cured EPDM elastomers may fail.
What is needed is a new method to cure such elastomers, that result in cured or vulcanized products (e.g. seals) that have improved thermal stability and higher cure temperatures.